Device and method of detecting ferrite and non-ferrite objects

ABSTRACT

A metal detector for detecting the presence of a ferrite object in the proximity of inductively coupled sensor having overlapping D shaped transmitter and receiver coils. The metal detector has a phase shift circuit to phase shifting a sensor output signal by a known amount and a switch operating in synchronisation with an excitation signal of the sensor for sampling the amplitude of the phase shifted output signal.

BACKGROUND TO THE INVENTION

1. Field of the Invention

The invention relates broadly to sensing devices for detecting ferriteand non-ferrite objects behind wall linings. Such a device is commonlycalled a metal and stud detector or finder. More particularly, theinvention relates to a device and method for detecting ferrite andnon-ferrite objects in proximity of an inductively coupled sensor.

2. Background Information

The use of inductively coupled sensors having a transmitter (orexcitation) coil and a receiver coil for detecting the presence of metalobjects is well known. The transmitter coil is excited with aperiodically varying excitation signal which produces and alternatingmagnetic field. The magnetic field induces a sensor signal in thereceiver coil. The presence of ferrite objects in proximity to the coilsaffects the inductive coupling between the coils. In particular, metalobjects within the proximity cause a phase shift between the excitationsignal and the induced sensor signal, which can be used to indicate thepresence of the metal object. Hitherto metal detectors employing thistype of sensor have suffered drawbacks including calibration stabilityand high processing demands on the detection circuit. To examine theamplitude and phase properties of received signals known devicestypically recorded the whole waveform and then calculate changes atevery point on the waveform. This creates a considerable processingoverhead that requires use of a powerful and expensive DSP typeprocessor.

In order to overcome problems with calibration stability known devicesmust be calibrated by a user calibration or automatic calibrationfunction immediately prior to each use. This makes manufacture and useof the device more complicated and introduces a calibration delay at thebeginning of each use of the device.

It is an objection of the present invention to provide a metal detectorand a method of detecting the presence of a metal object using aninductive type sensor that overcomes or at least ameliorates some or allof the above problems.

SUMMARY OF THE INVENTION

There is disclosed herein a metal detector for detecting the presence ofa ferrite object in the proximity of inductively coupled sensor having atransmitter coil and a receiver coil. The transmitter coil is excited byan excitation signal causing a sensor output signal to be induced in thereceiver coil. The metal detector has a phase shift circuit for phaseshifting the sensor output signal by a known amount and a switchoperating in synchronisation with the excitation signal for sampling anamplitude of the phase shifted output signal. A processor is connectedto an output of the sampling switch for determining the amplitude of thesampled output signal and displaying an indication of the amplitude on adisplay.

The metal detector also includes an electronic storage medium havingstored thereon calibration information for the sensor so that the metaldetector does not need to be calibrated before each use. By comparingthe output signal amplitude with the calibration information theprocessor can determine that distance of a ferrite or non-ferrite objectfrom the metal detector.

There is also disclosed herein a method of detecting the presence of aferrite or non-ferrite object in the proximity of an inductively coupledsensor as used in the metal detector.

Further aspects and disclosure of the invention are provided in and willbecome apparent from the following description.

BRIEF DESCRIPTION OF THE DRAWING

An exemplary form of the present invention will now be described by wayof example only and with reference to the accompanying drawings, inwhich:

FIG. 1 illustrates an inductive type metal sensor,

FIG. 2 illustrates a circuit diagram of a metal detector according tothe invention, and

FIG. 3 illustrates a phase shift circuit of the invention.

DESCRIPTION OF THE EXEMPLARY FORMS OF THE PRESENT INVENTION

Referring to the drawings, an inductively coupled sensor 1 of a metaldetector has a transmitter circuit 2 and a receiver circuit 3. Thetransmitter circuit 2 comprises a D shaped inductor coil 11 and acapacitor in parallel having a resonant frequency of 5.3 KHz. Thereceiver circuit 3 comprises an identical D shaped inductor coil 12 anda capacitor in parallel. The receiver circuit 11 and transmitter circuit12 are located with overlapping curved portions 13, 14 and areinductively coupled. When the transmitter coil 11 is excited with aperiodically varying (i.e. alternating) excitation signal an alternatingmagnetic field is set up that induces a resultant periodically varyingsignal in the receiver coil 13. The presence of a non-ferrite objectwithin the magnetic field of the transmitter coil 11 will cause anincrease in the amplitude of the resultant periodically varying signalgenerated in the receiver coil 13. The amount of increase is related tothe distance between the transmitter 111 and receiver 13 coils and thenon-ferrite object. The presence of a ferrite object within the magneticfield of the transmitter coil 11 will cause a change in the phase (i.e.a phase shift) of the resultant periodically varying signal generated inthe receiver coil 13. The amount of phase shift is related to thedistance between the transmitter and receiver coils and the ferriteobject.

A microprocessor 4 generates a 5.3 KHz square wave excitation signal forthe transmitter circuit 2, which in turn generates a 5.3 KHz alternatingmagnetic field. A 5.3 KHz sine wave signal is induced in the receivercoil 13 by the alternating magnetic field. This sine wave signal fromthe receiver circuit 3 is amplified by a pre-amp circuit 5 connected toan output terminal of receiver circuit 3. The pre-amp 5 is a typicalnon-inverting amplifier. The amplified sine wave signal is thenmanipulated and analysed by a series of following circuits to determinewhether there is a ferrite, or non-ferrite object in the vicinity of thetransmitter and receiver coils.

In practice there will always be a constant phase shift between thegenerated excitation signal and the sensor signal induced in thereceiver coil even in the absence of ferrite or non-ferrite objects inproximity of the sensor. This phase shift results from the physical andelectrical characteristics of the sensor and coils and imperfectcoupling of the coils. As discussed later, the sensor output signal issampled in synchronisation with the excitation signal. In order toeliminate the effect of the inherent phase shift, so that any phaseshift can be attributed to the presence of a ferrite object, a phaseshift circuit 6 is provided to move the sensor output sine wave to aspecific reference phase, effectively eliminating the effect of theinherent phase shift in later sampling. In the preferred embodiment thephase shift circuit 6 shifts the phase of the output sine wave 15 sothat its peak amplitude is in a sampling window 16 of a set of samplingswitches 8 when no external ferrite or non-ferrite objects are inproximity of the sensor. The sampling switches are driven insynchronisation with the excitation signal at 5.3 KHz. A preferredembodiment of the phase shift circuit is shown in FIG. 3. The op-ampcircuit is an all-pass filter that alters the phase of the sine wave 15without affecting its amplitude. In the circuit phase shift varies withfrequency. The sine wave 15 has a constant frequency of 5.3 KHz and sothe phase shift is constant. The amount of phase shift can also bevaried by changing variable resistor R1. The value of R1 is determinedduring manufacture to eliminate the effect of the inherent phase betweenthe sensor coils 11 and 13 due to electrical characteristics of thesensor and coils and imperfect coupling.

The output of the switches 8 is a regulated DC output voltage.Thereinafter, if a non-ferrite object comes into proximity of the sensorthe amplitude of the receiver coil signal increases and the regulated DCvoltage output of the switches likewise increases. If a ferrite objectcomes into proximity of the sensor the phase of the receiver coil signalchanges moving the peak amplitude out of the sampling window of theswitches 8 and the regulated DC voltage output of the switches thereforedecreases. This arrangement allows amplitude to indicate the presence ofboth ferrite and non-ferrite objects in proximity of the sensor and so aconventional 8-bit microprocessor can be used in the metal detectorrather that a more expensive DSP chip that was hitherto needed to detectphase shift caused by ferrite objects.

A DC bias (offset) voltage 7 is applied on the output of the phase shiftcircuit 6 in order to keep its peak output voltage above a minimumvalue, for example 0.5 volts. There is no need to calibrate for a nullsignal because the aim of the DC bias voltage is to compensate thecircuit for inductor coil and capacitor tolerances of the transmitterand receiver circuits.

A power amp 9 is used to amplify the output DC voltage of the switches 8to achieve a higher resolution of data analysis. The amplified DCvoltage is sampled by a 10 bit A/D converter 10 that is coupled to themicroprocessor 4 for comparison of the DC level with reference data anddetermination of the presence and distance of an object in the proximityof the sensor.

The relative position of the transmitter and receiver coils is fixed andknown and so the distance of any object from the sensor can bedetermined from the amount of change in amplitude of the regulated DCoutput voltage of the switches 8. The metal detector is calibrated atthe factory to determine the change in amplitude of the regulated DCoutput voltage of the switches 8 when ferrite and non-ferrite objectsare brought into proximity of the sensor. The calibration information isstored in an EEPROM (Electrically Erasable Programmable Read-OnlyMemory) for lookup by the microprocessor. The metal detector does notneed to be calibrated before each use and no user or automaticcalibration functions are provided in the metal detector. Simply bycomparing the regulated DC output voltage of the switches 8 with thecalibration information is stored in the EEPROM the processor canprovide an information to the user on the type of any object beingdetected, i.e. ferrite or non-ferrite, and its distance from the sensor.By displaying distance information on the output display the user canlocate the object behind a wall lining and determine its depth withinthe wall/behind the wall lining. As the metal detector is moved over awall surface in a single direction the displayed distance of an objectfrom the detector will reduce to the sensor until the detector passesover the object after which the displayed distance will begin toincrease. At the closest distance the object is directly under thesensor and the displayed distance if the objects depth.

Other advantages of the metal detector are that it prevents drillingonto a metal object if a traditional calibration and calibrate cycle isdone on top of any metal object. It can also minimize inaccuracy ofdepth indication if calibrated near a ferrite or non-ferrite metal.

It should be appreciated that modifications and/or alterations obviousto those skilled in the art are not considered as beyond the scope ofthe present invention.

1. A metal detector for detecting the presence of a ferrite object inthe proximity of inductively coupled sensor having a transmitter coiland a receiver coil, the metal detector comprising a phase shift circuitfor phase shifting a sensor output signal by a known amount and a switchoperating in synchronisation with an excitation signal of the sensor forsampling an amplitude of the phase shifted output signal.
 2. A metaldetector comprising: a sensor circuit having a transmitter coil and areceiver coil inductively coupled to the transmitter coil, a phase shiftcircuit connected to an output of the receiver coil, the phase shiftcircuit having a fixed phase shift for phase shifting a receiver coiloutput signal by a known amount, a sampling switch connected to anoutput of the phase shift circuit and operating in synchronisation withan excitation signal of the transmitter coil, a processor connecteddirectly or indirectly to an output of the sampling switch fordetermining an amplitude of the sampled output signal, and a display fordisplaying an indication of the determined amplitude.
 3. The metaldetector of claim 2 further comprising a comparator or processorcomparison function for comparing the sampled amplitude to a referenceamplitude and wherein an indication of the comparison is displayed onthe display.
 4. The metal detector of claim 2 further comprising acomparator or processor comparison function for comparing the sampledamplitude to fixed calibration information and wherein an indication thecomparison is displayed on the display.
 5. The metal detector of claim 2which is factory calibrated and which has no user or automaticcalibration function for calibrating the metal detector during use.
 6. Ametal detector for detecting the presence of a ferrite or non-ferriteobject in the proximity of an inductively coupled sensor having atransmitter coil and a receiver coil by detecting a phase or amplitudedifference between signals in the coils, wherein the metal detectorincludes an electronic storage medium having stored thereon calibrationinformation for the sensor such that the metal detector does not need tobe calibrated before each use.
 7. The metal detector of claim 6 furthercomprising: a display for indicating a distance of a ferrite ornon-ferrite object from the metal detector, a sampling circuit forconverting a phase difference between signals in the coils into a changein a signal amplitude, and a processor coupled to the sample circuit,the electronic storage medium and the display and programmed to comparethe signal amplitude with the calibration information and output on thedisplay a distance of a ferrite or non-ferrite object from the metaldetector.
 8. The metal detector of claim 6 wherein the sampling circuitcomprises a phase shift circuit for phase shifting a sensor outputsignal by a known amount and a switch operating in synchronisation withan excitation signal of the sensor for sampling an amplitude of thephase shifted output signal.
 9. A method of detecting the presence of aferrite object in the proximity of an inductively coupled sensor havinga transmitter coil and a receiver coil, the method comprising: applyingan periodically varying excitation signal to the transmitter coil toinduce a periodically varying sensor signal in the receiver coil,manipulating the sensor signal so that the sensor signal has a knownphase reference, measuring the amplitude of the manipulated sensorsignal at a known point in its cycle, and displaying an indication ofthe measured amplitude.
 10. The method of claim 9 wherein the displayingan indication of the measured amplitude comprises comparing the measuredamplitude to a reference amplitude and displaying an indication of thecomparison.
 11. The method of claim 9 wherein the displaying anindication of the measured amplitude comprises comparing the measuredamplitude to fixed calibration information and displaying an indicationof the calibration information.
 12. The method of claim 9 whereinimmediately prior to the method there is no calibration step.
 13. Amethod of detecting the presence of an object in the proximity ofinductively coupled sensor having a transmitter coil and a receiver coilwherein an output of the sensor is phase shifted a known amount and isthen sampled in synchronisation with an excitation signal of the sensorto determine its amplitude.
 14. A method of detecting the presence of aferrite or non-ferrite object in the proximity of an inductively coupledsensor having a transmitter coil and a receiver coil by detecting aphase or amplitude difference between signals in the coils, the methodcomprising provided an electronic storage medium having stored thereoncalibration information for the sensor such that the metal detector doesnot need to be calibrated before each use.
 15. The method of claim 14further comprising: converting a phase difference between signals in thecoils into a change in a signal amplitude, compare the signal amplitudewith the calibration information, output on a display a distance of aferrite or non-ferrite object from the metal detector.
 16. The method ofclaim 15 wherein converting a phase difference between signals in thecoils into a change in a signal amplitude comprises manipulating thesensor signal so that the sensor signal has a known phase reference, andmeasuring the amplitude of the manipulated sensor signal at a knownpoint in its cycle.